1. Field of the Invention
The present invention relates to an image forming apparatus for printers, copy machines and the like which supply toner from a developing device to an electrostatic latent image formed on the surface of a photosensitive member so as to form an image.
2. Description of the Prior Art
A printer a having the construction shown in FIG. 1 is provided as an example of a conventional image forming apparatus using an electrophotographic copy process.
In this image forming apparatus, a photosensitive drum 1 has arranged around its periphery along the direction of rotation (direction of arrow "Y") a charger 2, developing device 3, transfer charger 4, separation charger 5, jam detection sensor 6 comprising light emitting element 7 and photoreceptor 8, cleaning device 9, and main eraser 10.
An optical device 11 incorporating laser generator 12, lens 13 and mirror 14 is disposed above photosensitive drum 1. A light beam LB emitted from laser generator 12 is projected onto the surface of photosensitive drum 1 between charger 2 and developer 3 through lens 13 and mirror 14.
Developing device 3 is a commonly known device which uses a composite developer comprising a carrier and toner. Developing device 3 provides a rotatably or fixedly mounted developing sleeve 15 having a magnetic roller 16 disposed therein opposite said photosensitive drum 1. A specific bias voltage Vb is applied to said developing sleeve 15 from a power source 20. Toner and carrier which are mutually charged with opposite polarities are maintained in the state of a magnetic brush on the outer periphery of said developing sleeve 15.
In the image forming apparatus of the aforesaid construction, images are formed as described hereinafter.
Photosensitive drum 1 rotates in the direction of arrow "Y" and the surface thereof is uniformly charged by means of a discharge from charger 2, and a light beam LB emitted from laser generator 12 is projected so as to form an electrostatic latent image thereon.
Subsequently, the aforesaid electrostatic latent image is developed by toner supplied by a developing device 3 disposed opposite. As shown in FIG. 2, the developing operation is based on the voltage difference between the surface potential V.sub.o of the photosensitive drum 1 and the bias voltage V.sub.b, wherein toner T.sub.o adheres only to the image portion 21 having a reduced potential induced by the projected light beam LB, while toner does not adhere to the background portion 22. On the other hand, the carrier on top of developing sleeve 15 is bound by the magnetic force of magnetic roller 16 and is thereby preserved without adhering to the surface of said photosensitive drum 1.
Toner T.sub.o on photosensitive drum 1 is then transferred to sheet "S" disposed opposite transfer charger 4, said sheet "S" is then separated from the surface of photosensitive drum 1 by separation charger 5, passes the fixing device (not shown in the drawings) and is discharged to a prescribed discharge portion (also not shown in the drawings). The surface of photosensitive drum 1 which has passed opposite separation charger 5 then passes opposite the jam detection sensor 6, and continues to rotate until opposite cleaning device 9 and eraser lamp 10 whereupon the residual toner and residual charge are removed respectively, thereby preparing said surface of photosensitive drum for a subsequent image forming process.
In the conventional image forming device, sheet "S" at times does not separate from the surface of the photosensitive drum 1 at the separation portion, but rather continues to be carried in a state wherein it remains covering the surface of said photosensitive drum 1. If the aforesaid situation is not prevented, photosensitive drum 1, cleaning device 9 and eraser lamp 10 will be damaged, and removing the paper jam is extremely difficult.
Therefore, the power supplied to charger 2 is interrupted when sheet "S" is detected by jam detection sensor 6, and the drive unit (not shown in the drawing) which drives photosensitive drum 1 is immediately stopped, as shown in FIG. 7. Further, switch 17 in power unit 20 is switched from developing bias power source 18 to ground 19, the bias voltage of developing sleeve 15 drops to nearly 0 V, and the operator is safe from electrical shock even if he should touch the developing sleeve 15 when servicing the paper jam.
The aforesaid process does not only occur when the sheet "S" does not separate from the photosensitive drum in the separation portion thereby producing a paper jam. That is, the drive unit is stopped and the power unit switched also when a jam occurs after the sheet "S" is transported out of the separation portion and into the transport portion, when the charging means leaks or is disconnected, when the main eraser is switched OFF, and when the operator opens the cover of the printer or copy machine to access the interior of said machine.
However, when switch 17 is switched, the developing bias V.sub.b simultaneously drops to nearly 0 V, while electric potential maintained in the region "X" on the surface of photosensitive drum 1 between charger 2 and developing device 3 does not drop immediately, but rather tends to decrease slowly.
However, photosensitive drum 1 continues to rotate due to the influence of inertia even after the drive unit is stopped. Thus, when a jam occurs while a charge is maintained in the region "X" of photosensitive drum 1, each time said charged portion 22 passes opposite the developing sleeve 15 some carrier is released from the restraint of magnetic roller 16 thereby separating from the developing sleeve 15 and adhering to the charged portion 22 due to the electrical force produced by the difference in potential between the surface potential V.sub.o ' of the charged portion and the developing bias voltage V.sub.b (which is approximately 0 V), as shown in FIG. 4.
There is also a type of image forming apparatus divided into dual vertical units wherein a developing sleeve 15 travels along the outer periphery of a photosensitive drum 1 whenever the top unit is opened relative to the bottom unit. When the top unit is opened for paper jam processing, the carrier is lost from the developing sleeve 15 in the manner previously described.
A disadvantage obtains in that the carrier is therefore removed from the developing sleeve 15 each time a jam occurs, and since said carrier cannot be gradually replenished, unlike the toner, the longevity of said composite developer is shortened.
Further, following the completion of the jam processing, a surface charge remains on the upstream side of the image portion of the photosensitive drum 1, and the same aforesaid problem occurs each time the photosensitive drum 1 is again rotated after a temporary halt in said rotation.
The disadvantage of consumption of a carrier incorporated in bicomponent developer with each paper jam has been described above. However, this disadvantage is not limited only to bicomponent developers, but is also relevant to monocomponent developers as well. That is, adhesion of the monocomponent developer to the surface of the photosensitive drum whenever a jam occurs also wastes toner.